You can also search for competitors 'in the marketplace' on Amazon, Google, at physical locations, or on app stores.
Awareness + Promotion
Identify online communities where your customers "live" online. Message boards, websites, Facebook Groups, LinkedIn, Instagram influencer accounts, etc.
Actual distribution channels
By looking at the marketplace, how are other competitors distributing their service or product?
You can search for distributor and wholesale suppliers by searching 'industry keyword' and 'distributor/wholesaler' in Google.
The Census provides some information on sales channels in their Product by Industry table. Use the 'Codes' option to filter by product type and industries reported. This can give you an idea of different retail channels.